Thanks for the responses. As I mentioned, I am a new player and I don't really spend too much money; maxing out on evos is not going to happen anytime soon. If it's an efficient use of time and resources at my stage (to become stronger quicker so that I can clear the tower and pvp sooner), I'm willing to do it. Plus, I think the 2x XP event is running concurrently.

You're better off stocking up your stamina for tomorrow's event, which is the bonus materials one. Scoop up all the pyre embers, evil purple crystals and brimstone you can, since you will need it all later. As for the celestials, craft or buy them with honor/tower points. If there's an bigger event going on, like the pvp one that was running until today, you can also buy monarchs from event tickets that you earn. (Obviously don't do this if there's a juicy rune you can buy with those tickets instead.)

Not sure if materials is more relevant more for late game or early game as right now I don't see any of my heroes needing it for the 1st ascension. I rather get stronger now as it should make my mid to late game farming more flexible (more dungeon options available) and efficient. It should also allow me to be have an easier time tackling pvp and the tower in the short-term.

It's more relevant to late game, but "evo extravaganzas" are anything but. The drop rate for cels is tripled, sure, but the normal drop rate is so low (something like 1%) that it's not like they spring up everywhere. I feel that it's a bad idea to put stamina into this event unless you're really eager to level up quicker. When it comes to ascension, the vast majority of my celestials come from crafting/buying them.

Once you hit level 30 and challenge mode dungeons are unlocked, you'll want to turn your attention to crafting some improved runes to rare quality, which will give you a good power boost and put you in a good place for PvP and the Tower. This will also stand you in good stead for extracting all the gems from the campaign ASAP. You'll need various mats for these runes.